KINDS OF ROCKS. 67 



at one end of the series, and hard sandstones called quartzyte, hard 

 slates like roofing-slate, and partially crystallized limestones, at the 

 other ; so that a distinct line between them and the sedimentary beds 

 cannot always be drawn. 



The common ingredients are quartz, feldspar of different kinds, 

 mica, hornblende, pyroxene, talc, epidote, chlorite, serpentine ; to which 

 garnet, andalusite, staurolite, tourmaline, topaz, graphite may be added 

 as characterizing a number of varieties. The rocks are aggregates in 

 general of two or more of the above-mentioned minerals ; and, as the 

 proportions may vary indefinitely, the kinds of rocks are not well 

 defined ; they m&y imperceptibly graduate into one another. 



Metamorphic rocks may, for the most part, be distributed into 

 three series parallel with one another. These are the mica-bearing 

 series, containing granite, gneiss, mica schist, etc. ; the homblendic, 

 characterized by the presence of hornblende or •the allied pyroxene, 

 as in syenyte, hornblendic gneiss, etc. ; and the hydrous magnesian 

 series, containing talc, chlorite, and serpentine rocks. Besides these, 

 there arc other groups, which, with the foregoing, are described beyond 

 in the following order : — 



1. Mica-bearing series. 



2. Hornblendic series. 



3. Felsitic, epidotic, and garnet rocks, having the mass or body of the rock compact 

 (cryptocrystalline). 



4. Hydrous magnesian series. 



5. Hydrous aluminous series, or rocks consisting essentially of agalmatolite or 

 pyrophyllite. 



6. Quartz rocks. 



7. Iron-ore rocks. 



1. The Mica-bearing Series. 



The mica-bearing series commences with granite, the most highly 

 crystalline, and descends through gneiss and mica schist to argillyte 

 or roofing slate, and also to quartzyte, which is but little removed from 

 a sandstone. Quartz is a constant ingredient, as well as mica. The 

 series branches off into crystalline feldspathic rocks like granulyte, 

 containing little or no mica. The specific gravity is between 2*4 

 and 2-8. 



(1.) Granite. — A granular crystalline rock, consisting of quartz, 

 feldspar, and mica, having no appearance of layers in the arrangement 

 of the mica or other ingredients. The mica is in scales, usually white, 

 black, or brownish, easily separable into thinner elastic scales by 

 means of the point of a knife ; the quartz is usually grayish white, 

 glassy, and without any appearance of cleavage ; the feldspar is com- 

 monly whitish or flesh-colored, less glassy than the quartz, and 

 showing a flat, polished cleavage surface in one or two directions. 



